Photographic method for printing viewing-screen structure including treatment of exposed coating with ammonium compound

ABSTRACT

Method comprises (a) applying to a supporting surface for a viewing-screen structure of a cathode-ray tube a coating comprised of a dichromate-photosensitized organic binder, (b) exposing the coating of a pattern of actinic energy, including both heat energy and ultraviolet energy, (c) wetting the exposed coating with a dilute aqueous solution of an ammonium compound, such as ammonium oxalate, (d) and flushing the coating with an aqueous solvent to remove those portions of the coating that have been hardened solely by heat energy, while retaining portions of the coating that have been hardened by ultraviolet energy.

United States Patent [1 1 Baker June 3, 1975 PHOTOGRAPHIC METHOD FORPRINTING VIEWING-SCREEN STRUCTURE INCLUDING TREATMENT OF EXPOSED COATINGWITH AMMONIUM COMPOUND [52] US. Cl 96/36.1; 96/4.8 R", 96/49; 96/36;117/335 R; l17/33.5 C; l17/33.5 CM

[51] Int. Cl G036 5/00 [58] Field of Search 96/361, 48 R, 49, 36;117/335 CM, 33.5 C, 33.5 R

[56] References Cited UNITED STATES PATENTS 9/1972 Kawamura 96/3611/1973 Nishizawa et al.... 96/361 1/1973 Rohrer et al 96/361 3,793,0352/1974 Patel et a1 96/361 Primary Examiner-Norman G. Torchin AssistantExaminer-Edward C. Kimlin Attorney, Agent, or Firm--G. l-l. Bruestle; L.Greenspan [57] ABSTRACT Method comprises (a) applying to a supportingsurface for a viewing-screen structure of a cathode-ray tube a coatingcomprised of a dichromatephotosensitized organic binder, (b) exposingthe coating of a pattern of actinic energy, including both heat energyand ultraviolet energy, (c) wetting the exposed coating with a diluteaqueous solution of an ammonium compound, such as ammonium oxalate, (d)and flushing the coating with an aqueous solvent to remove thoseportions of the coating that have been hardened solely by heat energy,while retaining portions of the coating that have been hardened byultra- 10 Claims, No Drawings PHOTOGRAPHIC METHOD FOR PRINTINGVIEWING-SCREEN STRUCTURE INCLUDING TREATMENT OF EXPOSED COATING WITHAMMONIUM COMPOUND BACKGROUND OF THE INVENTION This invention relates toa novel photographic method for printing a viewing-screen structure,particularly for an apertured-mask-type color-television picture tube.

A commercial color-television picture tube of the apertured-mask type isa cathode-ray tube which includes a screen structure comprised of amultiplicity of red-emitting, green-emitting and blue-emitting phosphorelements on the inner surface of the viewing window of a faceplate panelof the tube. An apertured mask (also called shadow mask), positioned inthe panel in closely spaced relation with the phosphor elements, aids inselectively exciting the phosphor elements.

In order to make a television picture with suitable resolution and colorpurity, the process of forming the phosphor elements must be capable ofproducing a very large number of phosphor elements of relatively smalland uniform size which are accurately positioned with respect to oneanother. In one process for printing the phosphor elements, the innersurface of the viewing window is coated with a mixture comprised ofphosphor particles, polyvinyl alcohol, and a dichromate sensitizer suchas ammonium dichromate for the polyvinyl alcohol. Actinic energy isprojected from a small area source through the apertured mask incidentupon the coating. The mask functions as a photographic negative (orpositive) to transmit a pattern of energy, which produces in the coatingregions with greater solubility and regions with lesser solubility. Theexposed coating is developed by flushing with an aqueous solvent, untilthe more soluble regions of the coating are removed by solvent action,leaving the less soluble (hardened) regions adhered to the supportingsurface.

It is known that the size and thickness of the phosphor elements aredetermined in part by various parameters including the geometry of theparts of the system, the constitution of the coating, the amount of heatand light to which the coating is exposed, and the development of theexposed coating. Generally, the greater the total amount of heat andultraviolet energies employed to expose the coating, the larger will bethe phosphor elements formed. Since the phosphor elements are closelyspaced, variations in the size of the phosphor elements may lead toproblems in the operation of the tube. Overexposed portions of thecoating may produce oversized elements which may overlap adjacentelements, producing color impurity in the video picture. Underexposedportions may produce elements which do not adhere to the supportingsurface or may produce undersized elements which may result in poorcolor purity and/or reduced brightness in the video picture. Theexposure or total energy per unit area is the summation of theincremental actinic energies including heat energy and ultravioletenergy, applied to the coating.

There is considerable variation in the exposure across the field ofasingle viewing screen. Some regions are affected by heat energy alone,and some regions are affected by a combination of heat energy andultraviolet energy. This difference may occur in several ways;

for example, over the entire image by a flood exposure of either heat orultraviolet; at the image edges by a penumbra produced by light from afinite light source projected through a mask or stencil, or bydistortions or aberations in the optical system, or by light scattering(diffusion) in the coating. In any of these instances, it may bedesirable during development to distinguish between those regions whichhave been affected by ultraviolet energy or a combination of ultravioletenergy and heat energy and those regions that have been affected by heatenergy alone or have not been affected at all.

SUMMARY OF THE INVENTION The novel method for printing a viewing-screenstructure comprises (a) applying to a supporting surface a coatingcomprised of a dichromatizable organic binder such as polyvinyl alcohol,a dichromate photosensitizer therefor and, optionally, particles ofscreenstructure material; (b) exposing the coating to a pattern ofactinic energy including both heat energy and ultraviolet energy; (c)wetting the exposed coating with a dilute aqueous solution of anammonium compound, such as ammonium oxalate; and (d) developing saidcoating by flusing the coating with an aqueous solvent to remove themore soluble portions and those less soluble portions of said coatingthat have been hardened (rendered less soluble) by heat energy alone,while retaining those less soluble portions that have been hardened(rendered less soluble) by ultraviolet energy.

By wetting the exposed coating with a dilute aqueous solution of anammonium compound prior to or during development of the coating,undesired portions of the coating that have been hardened by heat energyalone are removed during the development step. The novel method may beused for removing the undesired portions of the coating which werepreviously retained during development. Or, the exposure of the entirefield may be lengthened so that the entire coating is more fully exposedby ultraviolet energy, and yet, after development, the undesiredportions are removed. The novel method may thereby be used to improvethe adherence of the desired exposed portions of the screen structure.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENT Example Prepare a coatingcomposition of the following formulation:

125 grams blue-emitting, silver-activated zinc sulfide phosphor,

138 grams 10 weight percent aqueous solution of polyvinyl alcohol havingan average molecular weight of about 170,000 to 220,000,

11 grams 10 weight percent aqueous solution ammonium dichromate, and

268 grams deionized water. The coating composition is carefully mixedand the viscosity adjusted to be in the range of about 10 tocentipoises. The formulation is then flow coated upon the inner surfaceof the faceplate of a 23-inch rectangular color-television picture tubeand dried. The apertured mask for the faceplate is then inserted in themounts provided therefor and the faceplate assembly positioned upon alighthouse platform. Ultraviolet light from the small-area light sourcein the light house is projected through the mask which permits a patternof light to fall incident upon and to expose the coating for about 11minutes. The faceplate panel is then removed from'the lighthouse and themask removed from the faceplate panel. The exposed coating is thenwetted with a 0.5 weight percent aqueous solution of ammonium oxalateapplied by spraying or fogging the solution onto the coating. With thecoating still wet and preferably within about seconds of the completionof applying the ammonium oxalate solution, the coating is developed byflushing with water to remove the still soluble portions of the coatingtogether with those portions of the coating that have been insolubilizedsolely by heat. Those portions of the coating that have beeninsolubilized by ultraviolet light or a combination of heat andultraviolet light are retained in place. By applying an aqueous ammoniumoxalate solution to the coating, normal-sized phosphor dots aredeposited with about 150 percent of the light exposure normallyemployed. However, the adherence of the dots is markedly improved withsubstantially no change in dot size.

GENERAL CONSIDERATIONS The novel method may be used to print variousviewing-screen structures by a photographic process. Dot screens andline screens are examples. By viewingscreen structure" is meant anycomponent part of a viewing screen for a display device; for example, aluminescent layer, or a light-absorbing layer for a cathode-ray-tubetarget. The novel method may be used to expose a layer comprised of amixture of photobinder and particles, or to expose a layer of a clearphotobinder, and then phosphor particles or light-absorbing particlesmay be deposited on the exposed areas. In the example, a phosphor-screenstructure is deposited directly by exposing a layer comprised ofphosphor particles mixed with a photobinder. An alternative method is toexpose a layer of clear photobinder, then deposit phosphor particlesthereon and then remove the more soluble portions of the photobinder andthe overlying and/or embedded phosphor particles. Another method forpreparing a phosphor-screen structure is to expose a layer of clearphotobinder, remove the more soluble portions thereof, then depositphosphor particles thereover and finally remove the less solubleportions of the photobinder layer with the phosphor particles thereonand/or therein, leaving phosphor particles in the portions previouslyoccupied by the more soluble portions of the photobinder layer. Thenovel process may also be used to produce nonluminescent screenstructures, such as a light-absorbing matrix for example, as describedin US. Pat. No. 3,558,310 to E. E. Mayaud. An example of this method isto expose a clear photobinder layer to a light image, to remove the moresoluble portions thereof, then to deposit light-absorbing particles suchas fine-particle graphite thereover, then to remove the less solubleportions of the photobinder layer with the light-absorbing particlesthereon, leaving the light-absorbing particles in the portionspreviously occupied by the more soluble portions of the photobinderlayer. Thus, the novel method may be used to photodeposit eitherluminescent or nonluminescent screen structures.

The example illustrates the use of the novel method to prepare a screenfor a color-television picture tube by the slurry process. Many suitablecoating formulations for the novel method applied to the slurry processare described in US Pat. No. 3,269,838 to T. A. Saulnier, .lr.Generally, the coating formulation is comprised of a dichromatizableorganic binder, a dichromate photosensitizer for the binder, andparticles of screen-structure material. The organic binder may be anorganic colloid, such as gelatin or glue, but is preferably polyvinylalcohol having a molecular weight greater than about 120,000. The bindermust be dichromatizable; that is, capable of being renderedphotosensitive with dichromate ions. The photosensitizer may be anysoluble dichromate such as sodium dichromate, potassium dichromate, orammonium dichromate. The photosensitizer may be present in amounts of 2to 20 weight percent of the weight of water-soluble polymer present. Thescreen-structure material may be, for example, luminescent material,inert material, or light-absorbing material, which is to bephotodeposited as a screen structure. The coating may, if desired,include a resin, such as an acrylate copolymer, which is not renderedphotosensitive by dichromate ions.

The coating is exposed to any actinic radiation such as infrared rays,visible light rays, or ultraviolet light rays. Suitable exposureequipment is described in the art, such as the lighthouse described inUS. Pat. No. 3,592,112 to Harry R. Frey, which employs a collimator. Asimilar lighthouse which does not employ a collimator may be used, andsuch a lighthouse produces similar effects with shorter exposures.Significant qualities of the exposure are that it includes both heatenergy and ultraviolet energy, and that the coating regions that areinsolubilized by these energies are not identical. The normal necessaryexposure is determined empirically to provide developed exposed areas ofa prescribed size with adequate adherence to the supporting surface.With significantly longer exposures; that is, with about to 200 percentof the normal necessary exposure, the exposed areas after developmentare oversized and frequently overlap on one another.

After exposure, the coating is wetted with a dilute aqueous solution ofan ammonium compound; such as ammonium hydroxide; a salt of an organicacid, such as ammonium oxalate, ammonium formate, ammonium citrate,ammonium acetate; or a salt of an inorganic acid, such as ammoniumchloride, ammonium nitrate; and ammonium carbonate. By dilute" is meantconcentrations of about 0.01 to 2.0 weight percent and preferably about0.5 to 1.0 weight percent of the aqueous solution. The solvent in thedilute aqueous solution of ammonium compound may consist only of wateror may include a portion of alcohol or other nonaqueous liquid that ismiscible in water.

The effect of the ammonium compound solution is to act on the chromiumions in the reverse manner of a short flood actinic infrared exposure ofthe coating. This is believed to cause some trivalent chromium ions inthe coating to be converted back to hexavalant ions. The ammoniumcompound solution dissolves only the heat-hardened coating and, unlikedilute solutions of oxidizing agents, does not attack theultraviolethardened coating. Thus the novel method is selflimiting inits action on the coating. By using a dilute solution of an ammoniumcompound, the effect may be parameters in the method. This may requirelonger exposures to be used.

Exposure techniques that use shadowing (as in the example) or projectionusually have a penumbra along the margins of the lighted areas of thepattern of actinic radiation. Usually, this marginal region in thecoating is principally heat hardened and may be advantageously acted onby the novel method without adversely affecting the ultraviolet-exposedor unexposed regions. The size of the penumbra may be enlarged byemploying larger diameter collimators (e.g., 150 to 200 mils) in thelighthouse, and may be reduced by employing smaller diameter collimators(e.g., 80 to 140 mils). Where a short flood exposure is employed toimprove adherence, the novel method may be used to remove anyunderexposed coating material in the regions intended to be unexposed.

The solution may be applied to the coating by fogging, spraying,dipping, soaking, flushing, pouring, or other application techniques.The action of the ammonium compound solution may be almost self-limitingin another respect. Where the solution is applied to a dry coating, thecoating takes up a limited amount of solution up to saturation. Anyexcess solution beyond this has essentially no effect on the exposedcoating. The solution may be applied as a separate step or as theinitial portion of the development step.

The time that the ammonium solution is permitted to act on the exposedcoating is not critical. Time variations may, however, cause minorvariations in the effectiveness of the treatment. it is preferred thatthe development step start about seconds or less after the completion ofthe wetting step. This permits suitable process control on automaticallyoperating production equipment.

The development of the wet coating is conducted in the usual way byflushing the coating with an aqueous solvent, which may be deionizedwater with or without additives present. The flushing may be by dippingor spraying or other method of application which will carry away theunexposed areas of the coating and leave the exposed areas in place.

The following are some solutions, in addition to the ammonium oxalatesolution disclosed in the example, of nonoxidizing, nonreducing ammoniumcompounds which may be used in the novel method.

Ammonium Formate Solution Dissolve at room temperature 3.0 grams ofchemically pure ammonium formate in 997 grams of demineralized water.The pH of the solution should be about 5.0 to 6.0.

Ammonium Citrate Solution Dissolve at room temperature 4.0 grams ofchemically-pure grade ammo nium citrate in 996 grams of demineralizedwater. The

pH of the solution should be about 5.0 to 6.0.

Ammonium Nitrate Solution Dissolve at room temperature 6.0 grams ofreagent-grade ammonium nitrate in 994 grams of demineralized water. ThepH of the solution should be about 4.0 to 5.0.

Ammonium Hydroxide Solution Mix at room temperature 200 grams ofreagent-grade 10% ammonium hydroxide with 800 grams of demineralizedwater. The pH of the solution should be about 10.0 to 11.0.

Ammonium Chloride Solution Dissolve at room temperature 8.0 grams ofreagent-grade ammonium chloride in 992 grams of demineralized water. ThepH of the solution should be about 4.5 to 5.5.

There are patents in the photographic art which disclose processes whichbear a resemblance to the novel method, but which are distinguishable.For example, U.S. Pat. No. 3,706,558 to H. R. Frey discloses a processcomprising applying a dilute solution containing a mild reducing agent,such as hydroquinone, to an exposed coating comprised of a dichromatedcolloid, such as dichromated polyvinyl alcohol; and then developing thecoating. In the Frey process, the reducing agent acts on the partiallyhardened regions of the coating to insolubilize them so that they areretained during development. In the novel method, the ammonium compoundsolution is not reducing and does not insolubilize partially hardenedregions of the coating, but acts to solubilize coating portions thathave been hardened solely by heat. U.S. Pat. No. 3,558,310 to E. E.Mayaud discloses a process comprising developing an exposed coatingcomprised of dichromated polyvinyl alcohol; and then treating thedeveloped image with a dilute aqueous solution of a chemically digestiveagent, such as hydrogen peroxide, to erode the edges of the retainedcoating regions. In the Mayaud process, all of the disclosed chemicallydigestive agents are oxidizing to the coating and erode the fullyhardened portions of the coating, albeit at a slower rate. The ammoniumcompound solutions of the novel method are not oxidizing and do noterode the fully hardened portions of the coating, but act only on thosecoating portions that have been hardened solely by heat.

I claim:

1. A photographic method for printing a viewingscreen structure upon asupporting surface comprising:

a. applying to said supporting surface a coating comprised of adichromatizable organic binder and a dichromate photosensitizer for saidbinder,

b. exposing said coating to a pattern of actinic energy, said exposureincluding both heat energy and ultraviolet energy,

0. wetting said exposed coating with a dilute aqueous solution of anammonium compound,

(1. and then flushing said exposed coating with an aqueous solvent toremove the more-soluble portions of the coating and those less-solubleportions of said coating that have been exposed solely to heat energy,while retaining in place the lesssoluble portions of said coating thathave been exposed to ultraviolet energy.

2. The method defined in claim 1 wherein said solution is a 0.01 to 2.0weight-percent solution of an ammonium compound selected from the groupconsisting of ammonium hydroxide, ammonium oxalate, ammonium formate,ammonium citrate, ammonium acetate, ammonium chloride, ammonium nitrateand ammonium carbonate.

3. The method defined in claim 1 wherein said dichromatizable organicbinder is a polyvinyl alcohol.

4. The method defined in claim 3 wherein said ammonium compound is asalt of an organic acid.

5. The method defined in claim 3 wherein said ammonium compound is asalt of an inorganic acid.

6. A method for printing a phosphor-viewing-screen structure for acathode-ray tube upon a supporting surface comprising:

a. applying to said supporting surface a coating comprised of polyvinylalcohol, a dichromate photosensitizer for said alcohol, and particles ofphosphor for said screen structure,

b. projecting a pattern of actinic energy upon said coating, saidexposure including both heat energy and ultraviolet energy, therebyreducing the solubility of the exposed portions of said coating,

c. wetting said exposed coating with a dilute aqueous solution of anammonium compound,

(1. and then flushing said wet coating with an aqueous solvent to removethe unexposed more-soluble portions of said coating and the less-solubleportions of said coating that have been exposed solely to 10 heat energywhile retaining in place the less-soluble portions of said coating thathave been exposed to ultraviolet energy.

7. The method defined in claim 6 wherein said ammonium compound is thesalt of an inorganic acid and is present in concentrations of 0.01 to2.0 weight percent of said aqueous solution.

8. The method defined in claim 6 wherein said ammonium compound is thesalt of an organic acid and is present in concentrations of 0.01 to 2.0weight percent of said aqueous solution.

9. The method defined in claim 6 wherein said ammonium compound ispresent in said solution in concentrations of about 0.1 to 0.5 weightpercent.

10. The method defined in claim 6 wherein step (d) is carried out within10 seconds after the completion of step (c).

1. A PHOTOGRAPHIC METHOD FOR PRINTING A VIEWING-SCREEN STRUCTURE UPON ASUPPORTING SURFACE COMPRISING: A. APPLYING TO SAID SUPPORTING SURFACE ACOATING COMPRISED OF A DICHROMATIZABLE ORGANIC BINDER AND A DICHROMATEB. EXPOSING SAID COATING TO A PATTERN OF ACTINIC ENERGY, SAID EXPOSUREINCLUDING BOTH HEAT ENERGY AND ULTRAVIOLET ENERGY, C. WETTING SAIDEXPOSED COATING WITH A DILUTE AQUEOUS SOLUTION OF AN AMMONIUM COMPOUND,D. AND THEN FLUSHING SAID EXPOSED COATING WITH AN AQUEOUS TION OF ANAMMONIUM COMPOUND, D. AND THEN FLUSHING SAID EXPOSED COATING WITH ANAQUEOUS SOLVENT TO REMOVE THE MORE-SOLUBLE PORTIONS OF THE COATING ANDTHOSE LESS-SOLUBLE PORTIONS OF SAID COATING THAT IN PLACE THELESS-SOLUBLE PORTIONS OF SAID COATING THAT HAVE BEEN EXPOSED TOULTRAVIOLET ENERGY.
 1. A photographic method for printing aviewing-screen structure upon a supporting surface comprising: a.applying to said supporting surface a coating comprised of adichromatizable organic binder and a dichromate photosensitizer for saidbinder, b. exposing said coating to a pattern of actinic energy, saidexposure including both heat energy and ultraviolet energy, c. wettingsaid exposed coating with a dilute aqueous solution of an ammoniumcompound, d. and then flushing Said exposed coating with an aqueoussolvent to remove the more-soluble portions of the coating and thoseless-soluble portions of said coating that have been exposed solely toheat energy, while retaining in place the less-soluble portions of saidcoating that have been exposed to ultraviolet energy.
 2. The methoddefined in claim 1 wherein said solution is a 0.01 to 2.0 weight-percentsolution of an ammonium compound selected from the group consisting ofammonium hydroxide, ammonium oxalate, ammonium formate, ammoniumcitrate, ammonium acetate, ammonium chloride, ammonium nitrate andammonium carbonate.
 3. The method defined in claim 1 wherein saiddichromatizable organic binder is a polyvinyl alcohol.
 4. The methoddefined in claim 3 wherein said ammonium compound is a salt of anorganic acid.
 5. The method defined in claim 3 wherein said ammoniumcompound is a salt of an inorganic acid.
 6. A method for printing aphosphor-viewing-screen structure for a cathode-ray tube upon asupporting surface comprising: a. applying to said supporting surface acoating comprised of polyvinyl alcohol, a dichromate photosensitizer forsaid alcohol, and particles of phosphor for said screen structure, b.projecting a pattern of actinic energy upon said coating, said exposureincluding both heat energy and ultraviolet energy, thereby reducing thesolubility of the exposed portions of said coating, c. wetting saidexposed coating with a dilute aqueous solution of an ammonium compound,d. and then flushing said wet coating with an aqueous solvent to removethe unexposed more-soluble portions of said coating and the less-solubleportions of said coating that have been exposed solely to heat energywhile retaining in place the less-soluble portions of said coating thathave been exposed to ultraviolet energy.
 7. The method defined in claim6 wherein said ammonium compound is the salt of an inorganic acid and ispresent in concentrations of 0.01 to 2.0 weight percent of said aqueoussolution.
 8. The method defined in claim 6 wherein said ammoniumcompound is the salt of an organic acid and is present in concentrationsof 0.01 to 2.0 weight percent of said aqueous solution.
 9. The methoddefined in claim 6 wherein said ammonium compound is present in saidsolution in concentrations of about 0.1 to 0.5 weight percent.